Rotating components used in jet engines and other high-speed machineries operate under large centrifugal stresses and can be fatigued through repeated use. For example, the Federal Aviation Administration (FAA) requires testing of newly-designed and revised engine hardware to establish life expectancy during the development phase of a new engine, and also when significant changes are made to an engine design.
Most jet engine manufacturers spend substantial time and money on computer simulations (xe2x80x9cfinite element modelsxe2x80x9d of the engine hardware) to obtain an initial estimate of the safe operating life of an engine part. It is impossible, however, to determine a rotor""s actual characteristics until it has been built and tested. Centrifugal fatigue life is generally measured at a centrifugal stress testing facility, in a spin test system designed to cycle the rotor from some low speed to operational speed then back again, alternately applying and relaxing the centrifugal stress.
Jet engines have numerous rotating parts that move and compress air (fans and impellers), or produce work (turbines). The elevated speeds at which these parts rotate induce high levels of centrifugal stress that tend to pull the components apart. A jet engine part such as a rotor usually fails in one of two ways. In the first failure mode, the rotor rotates to a speed that is sufficient to cause catastrophic material failure or burst. However, even when a rotor rotates at less than its burst speed, the rotor may eventually weaken over time as a result of many starts and stops. In this second failure mode, the part fatigues to a point where it develops a crack, which then grows to a critical size and ultimately causes the part to fail.
Typically, jet engine components such as rotors are thoroughly tested by the manufacturer as part of a development and qualification process to establish a safe operating life. The manufacturer will generally use a type of spin testing known as xe2x80x9cfatigue lifexe2x80x9d testing. Fatigue life is measured in cycles, with a run up to operating speed and back down to zero or some lower speed being counted as one cycle. Each cycle corresponds roughly to one takeoff and landing of an aircraft. After the designer has measured the number of cycles a part can withstand before a fatigue burst happens, safety and performance factors can be developed and applied. The safety factor determines how many cycles can be tolerated by an engine before a part must be replaced. The safety margin is established cooperatively by the engine manufacturer and the appropriate governing safety authority, and it is intended to assure that parts are replaced before there is any chance of burst in the engine.
Jet engine rotors are also routinely subject to periodic inspection after installation to determine the health of the rotor. To inspect an installed rotor, the engine is taken apart and the rotor is immersed in a fluorescent penetrant. The fluorescent penetrant will penetrate any cracks in the rotor and thereby facilitate their detection under ultraviolet light.
Moreover, methods are known for evaluating the health of a rotor by electronically monitoring vibrations. These methods generally require that the rotor be operated at a fixed speed to acquire information that will allow the phase angle of a crack to be determined. In one example, a method disclosed in U.S. Pat. No. 4,751,657, issued to Imam et al., requires that the engine rotor be held at several different speeds for successive measurements during engine run-up.
Establishing safe operating component lives is a critically important process, since the fragments of a bursting rotor cannot be contained by the engine casing. A rotor burst in flight would probably destroy the aircraft. The air transport industry has achieved its admirable safety record due in no small way to spin-pit life testing of engine parts; still, there have been some tragic accidents in air transport due to rotor burst. Examples of accidents traced to fatigue failure include the DC-10 crash at Sioux City; the in-flight separation of a propeller blade in the crash of an EMB-120 Embraer near Carrollton, Ga.; and the most recent fatal explosion of a fan disk assembly during take off of an MD-80 in Florida.
There is, therefore, still an unmet need for a technique which can accurately detect fatigue, cracks, and other anomalies in rotating components such as jet engine rotors and which is less cumbersome to use than fluorescent immersion or even electronic instruments that require controlled engine speed runs. Ideally, the technique could be used in a spin testing instrument used during engine design as well as for inflight instrumentation which might continuously monitor the health of a jet engine.
Described herein is a system for monitoring a rotor, such as a rotor in an operating jet engine, to detect cracks or other potentially hazardous conditions. The system can be used with great accuracy and sensitivity both to test a part in a centrifugal spin test facility as well as to test a part, in situ, during standard operation. Use of this system on an operating jet engine, or other machinery, can greatly reduce the risk of accidents, such as those described, above.
The system, itself, includes a vibration sensor for measuring the vibration of a rotor, a speed sensor for measuring the rotational speed of the rotor, a filter coupled to both the speed sensor and vibration sensor, and a signal processor coupled to the filter and speed sensor. The filter, which can be a digital filter, extracts a signal from the vibration measurement having a frequency synchronous with the rotation of the rotor. The processor is programmed to subtract a background (or baseline) vibration vector from the synchronous vibration signal to produce a vibration difference signal. The processor then measures and evaluates the amplitude, and preferably also the phase, of the vibration difference signal to determine if an anomaly, such as a crack, has developed.
In a preferred embodiment, the processor is programmed to evaluate the vibration difference signal by comparing it with the phase of a previous vibration difference signal to determine if the phases are consistent. The processor then subtracts a baseline vibration vector from the synchronous vibration signal and evaluates the difference to determine whether the difference exceeds a preset triggering limit. A crack is detected when the phase remains consistent and the difference between the synchronous vibration signal and the baseline vibration vector is greater than a triggering limit.
In a further preferred embodiment of the system, the rotor is a component of a jet engine on an aircraft.
A method of this invention may be broken down, as follows. The rotational speed and vibration of a rotor are measured. From the vibration measurement, a synchronous vibration signal having a frequency matching the frequency of rotation is filtered. A baseline vibration vector is subtracted from the synchronous vibration signal to produce a vibration difference signal. The amplitude, and preferably also the phase, of the vibration difference signal is then measured and evaluated to determine whether an anomaly, such as a crack, has developed. In a preferred embodiment of the method, the steps of the above-described method are repeated and vibration is measured while the rotor is accelerating or decelerating.
In another embodiment of the method of the invention, a mechanical component is rotated about an axis at a range of rotating speeds. A vibration signal value of the mechanical component is detected while the mechanical component is rotating. This vibration signal value is detected independently of whether, or at what rate, the mechanical component is accelerating or decelerating.